Here is a flavour, as tasteful as possible, of Paul McCarthy's excessive show at Tate Liverpool - a short account of a long video entitled Painter . The scene is an artist's studio mocked up in rickety plywood, with enormous brushes and tubes of paint as big as Claes Oldenburg sculptures. In a peroxide wig, a clown's nose, giant rubber hands and a hospital gown with nothing underneath, McCarthy plays the role of a demented abstract expressionist, spattering ketchup all over the place and probing the canvas with his phallic brush until it is thoroughly punctured.

The artist throws fits. He abuses his dealer. He warbles: 'De Koooooning, where are you?' with all the menace of Jack Nicholson in The Shining. Stuck for inspiration, he climbs inside one of the paint tubes. Bored, he paces back and forth like a captive primate. Time passes. Nothing gets painted.

Eventually, he turns on himself. Idly stabbing at one of his own hands with a cleaver, he manages to hack off a rubber finger. It takes some effort and concentration, through which he chortles and mutters. But finally the blade slices clean through and thick blood spurts out of the stump.

Painter is a good deal funnier, if more conventional, than most of McCarthy's videos. But it is entirely consistent in other respects with the Californian artist's performances: crude, carnal, embarrassing, ludicrous, sometimes painful, sometimes shocking to watch. McCarthy - or his penis - is almost always naked. His behaviour always involves compulsive repetition, escalating into wanton violence. His props rarely differ: joke-shop masks, daytime television sets, assorted condiments and clean tables that are invariably desecrated.

He operates at an infantile level - what do ketchup, mayonnaise or chocolate look like when smeared on the anus or dick? But he aims to penetrate the adult psyche: what does a raw sausage look like, severed and served on a plate? A McCarthy video is always an endurance test: for him, as he smears and abuses himself; for us as we sit it out to the bitter end, waiting for the cut-off, so to speak.

At 56, Paul McCarthy has long been a well-followed cult in America, first on the West Coast, more recently in Manhattan. The show at Tate Liverpool, a suspiciously last-minute stop on a lengthy tour, documents 30 years of performances through videos, photographs, soiled props and ill-treated sets. You see the young McCarthy lubricating himself with ketchup and walking through a mulch of broken glass. You see him in his forties, funnelling gunk into a polystyrene head, announcing: 'My daddy did this to me and you can do this to your son.' There are the animatronic figures of the 1980s - a man with his trousers down, rutting away at a tree; and the static sculptures, such as Spaghetti Man , half-rabbit, half-man, with a 40ft noodle for a penis.

There is McCarthy as a maniacal chef; an incestuous dad; a deranged Pinocchio administering unspeakable doses to a Pinocchio puppet. 'Because his portrayals of these personae are so convincing,' warn the curators, unnecessarily, McCarthy is wrongly assumed 'to be unhinged, clumsy and aggressive'. Heaven forbid that anyone would guess he is actually a professor at UCLA, addressing an exclusively art-world audience!

For who else would willingly sit through an hour of live McCarthy, cavorting in sauce and a mask? You can see them in the photographs from the Seventies, wearing loons and Yoko Ono specs, crammed into the lecture theatre with studious expressions on their faces as McCarthy slithers through the slime, masturbating with a plastic doll for a penis. It may have helped that they couldn't see his face, which is never clearly visible in the videos. Moreover, almost all of his early performances knowingly invoked the work of other artists. He creates a Barnett Newman zip by stuffing cotton wool between a pair of swing doors. He performs grand-guignol spectaculars like the Viennese Actionists, substituting ketchup for blood. He whips an oil-stained cloth against the wall, in a parody of action painting. He whips out his dick as a macho-expressionist paintbrush.

The ketchup was always significant in McCarthy's palette - fake, farcical, lubricious, yet a staple from the mall and the diner. It became crucial to his pop expressionism, as it's been called, along with the grotesque pastiches of children's shows, consumer television and Disney cartoons, degrading everything that Uncle Walt held dear. Heidi, Pinocchio, Bugs Bunny, Santa's elves: they all get screwed, sometimes quite literally, in these videos. A society constipated with its own fatuous greed is being given a salutary enema. Add to this the constant references to the art of the past and you have McCarthy's confrontational formula.

But no description of McCarthy's art as social critique really covers its more outlandish aspects. Most of his work is as wretched, narcissistic and low as it sounds. If you're thinking of taking teenagers - and the gallery recommends that they be accompanied - it may be an idea to skip the photographs of the pseudo-gynaecological operation, for all that it involves nothing much more than sausages, bagels and ketchup. But this is a relatively obvious gross-out, a purely physical shock, compared to his more psychologically unfettered performances which can linger in the mind, unresolved, probing to the quick of one's memory.

In Bossy Burger, McCarthy appears in an apron, chef's hat and Alfred E. Neumann mask (the freckled and gap-toothed boy from Mad magazine). Laid out on the table, like vessels on an altar, are the bottled ingredients for a cookery demonstration. The chef wills himself to start, muttering desperate exhortations beneath his breath - 'I love my work, I love my work' - but he soon begins to lose his place in the ritual. The chaos that follows is deeply alarming, veering between hysterical humour and unmediated dread. With its world turned upside down, its ludicrous masks, grotesque bodies and destabilising wit, Bossy Burger is peculiarly cathartic, a contemporary form of carnival.

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